In manufacturing semiconductor devices, in order to separate capacitors or elements, or otherwise to form contact holes, dry etching or film forming process using plasma has been performed on a substrate such as a semiconductor wafer (hereinafter, referred to as a “wafer”). As one of the apparatuses capable of performing such processes, a single wafer parallel plate type plasma processing apparatus is used, wherein a high frequency voltage is applied between an upper electrode and a lower electrode to generate plasma.
FIG. 9 shows a schematic diagram of the above-mentioned plasma apparatus. To describe it briefly, in an sealed chamber 1 which is a vacuum chamber, an upper electrode 11 (also serving as a gas shower head) and a lower electrode 12 (also serving as a substrate mounting table) are, respectively, disposed at an upper side and lower side thereof. A focus ring 13 made of, e.g., quartz is disposed to surround a wafer 100 on the lower electrode 12 (mounting table). An electrostatic chuck 14 which electrostatically attracts and holds the wafer 100 thereon has a foil-shaped electrode 15 therein to which a chuck voltage is applied from a power supply (not shown). Further, a specified processing gas selected according to a process type is discharged onto the wafer 100 from the gas shower head 11 (upper electrode) while simultaneously vacuum pumping the sealed chamber 1 via a vacuum pump 16 to maintain predetermined pressure therein. In the meantime, a high frequency voltage is applied between the upper electrode 11 and the lower electrode 12 from a high frequency power supply 17, whereby the processing gas is converted to plasma so that a specified process such as etching is carried out on the wafer 100.
At this time, after the processing gas discharged from the gas shower head 11 (upper electrode) reaches the vicinity of the surface of the wafer 100, the processing gas is exhausted past and below the periphery of the wafer 100 toward the bottom of the sealed chamber 1. Therefore, a gas flow of the processing gas near the peripheral portion (around the periphery) of the wafer 100 is different from that near the central portion of the wafer 100, thereby resulting in a discrepancy of a predetermined composition ratio in the processing gas between the peripheral portion and the central portion of the wafer 100. Further, parameters like impedance and conductance components between the plasma and the lower electrode 12 of the area where the wafer 100 is disposed are different from those of the outside the area. To elaborate, the dissociation degree of the processing gas is higher in the peripheral portion of the wafer 100 near an exhaust port compared to the central portion of the wafer 100. Since the plasma density is also higher in the peripheral portion of the wafer 100, the etching rate tends to be higher there compared to the central portion.
In response to the strong demands for forming devices even in areas near the periphery of wafer 100, to maximize the utilization of the wafer 100, superior in-surface uniformity of etching rate over the entire wafer surface needs to be achieved. Accordingly, a focus ring 13 formed of a conductor, a semiconductor or a dielectric material etc. is disposed to surround the wafer 100, thereby adjusting the plasma density of the regions above the peripheral portion of the wafer 100. Specifically, a material and an optimum shape of the focus ring 13 are selected according to the type of processing gas, the material type of etched film etc., when selecting a focus ring 13 suitable for the process.
Further, a method has been known for stabilizing plasma processing conditions between wafers such that a protection plate corresponding to the focus ring 13 is coated with a heating element (heater) and provided with a temperature sensor, and a temperature rise caused by the heating element is controlled according to the protection plate's temperature increase due to ion impact, thereby maintaining the protection plate at constant temperature (see, e.g., Reference 1).
[Reference 1] Japanese Patent Laid-open Publication No. H7-310187 (the end of paragraph 0010 and FIG. 5)
However, selection of a processing gas depends upon a film to be etched, and superior in-surface uniformity of a process performed on the entire surface of the wafer 100 is required in response to the recent trend of pattern miniaturization. Due to the uniformity requirement, it is becoming more difficult to choose the optimal material and shape of the focus ring 13 for controlling the plasma density near the peripheral portion of the wafer 100. Therefore, further effort is required to prevent the plasma density near the peripheral portion of the wafer 100 from becoming higher than that over the inner surface thereof.